1. Field of the Invention
The invention relates to management of driving power in a hybrid electric vehicle having two power sources whereby vehicle driveline vibration oscillations are attenuated.
2. Background Discussion
Powertrain configurations for hybrid electric vehicles have multiple power sources. A primary power source typically is an internal combustion engine, although other power sources, such as a fuel cell, can be used instead. A secondary power source would include an electric motor, such as an induction motor electrically coupled to a high voltage battery and an electric generator.
A hybrid electric powertrain may include a configuration in which multiple power sources distribute power to vehicle traction wheels through gearing of a transmission that defines divided power flow paths. The battery acts as an energy storing device for the generator and the motor as power is distributed through the divided power flow paths. The motor and the generator can be referred to as electric machines since either is capable of acting as a motor under certain driving conditions and as a generator under other driving conditions.
A vehicle system controller will maintain the vehicle at its maximum performance value by managing the power distribution among the various components of the vehicle powertrain. It manages the operating state of the engine, the generator, the motor and the battery to maximize total vehicle efficiency. The battery is an energy storage medium for the generator and the motor.
The vehicle system controller will ensure that power management for optimum vehicle performance and drivability is maintained as it commands desired engine torque and interfaces with a transmission control module. The transmission control module commands a motor torque to effect the commanded wheel torque. It also commands a generator torque to control the engine to achieve a desired engine speed.
For the purpose of disclosing the present invention, reference will be made to a so-called series-parallel powertrain configuration, although the invention is not limited to use in a powertrain configuration of that type. In a series-parallel powertrain configuration, a motor torque control must coordinate the wheel torque command and the generator torque command. It then must compensate for the inertia effect of the generator and the gearing elements to which the generator is connected. This will ensure that the vehicle system controller wheel torque command is fulfilled.
Environmental disturbances and powertrain dynamic interactions of the engine, generator and motor can result in oscillatory generator torque. An oscillatory generator torque can result in an oscillatory motor torque command, which can excite driveline torque oscillations. In turn, these driveline torque oscillations can result in oscillatory generator target speeds and actual generator speeds, which will cause oscillatory generator torque commands. A controlled coordination of the torque of the motor and the generator can potentially result in a so-called “positive feedback” control loop if the control system is not implemented properly. This positive feedback can cause driveline torque oscillations to be sustained.
Engine power available at the traction wheels needs to be estimated in order to afford the vehicle operator with maximum powertrain system capability under all engine and battery operating conditions without violating battery power limits. This engine power estimation is used to determine an appropriate maximum wheel power command, which is used to determine an optimum maximum desired wheel torque for a given set of operating conditions. When maximum powertrain system capability is demanded (e.g., in a full acceleration pedal startup, or at accelerator pedal commands with low battery limits), the engine power estimation directly affects the wheel power command and also the wheel torque command.
An implementation of an engine power estimation is done without exceeding allowable power limits as described in patent application Ser. No. 10/248,886, filed Feb. 27, 2003, entitled “Closed-Loop Power Control System For Hybrid Electric Vehicles”, now U.S. Pat. No. 6,991,053, issued Jan. 31, 2006. That patent application is assigned to the assignee of the present invention.
The engine power estimation is based on the difference between the commanded wheel power and the sum of the commanded motor power and the commanded generator power. Assuming that the transmission is delivering the commanded wheel power and wheel torque, the motor and generator torque commands can be oscillatory, as previously indicated, which may result in oscillatory motor and generator power values. This in turn may cause the engine power estimation to oscillate. This oscillatory engine power estimation can cause oscillatory wheel power commands, which would result in oscillatory wheel torque commands. Oscillatory wheel torque commands can amplify the driveline oscillations induced by the so-called “positive feedback” in the motor and generator coordination control.
Engine power can be estimated as disclosed in U.S. patent application Ser. No. 11/161,344, filed Jul. 29, 2005, which is entitled “Method for Estimating Engine Power in a Hybrid Electric Vehicle Powertrain”, now U.S. Pat. No. 7,285,869, issued Oct. 23, 2007. That application is assigned to the assignee of the present invention. Such engine power estimation actually is analogous to a coupling between the powertrain control module and the transmission control module, which can sustain driveline oscillations initiated by the transmission. These driveline oscillations are more noticeable during full accelerator pedal launches under low vehicle speeds and under operating conditions when battery power limits are low. They are less noticeable at relatively high vehicle speeds.